One sensitizer, a sterol, has previously been reported to be present in lanolin. In this study, a related substance, a sterol, with a molecular weight of 424 has been isolated. A few sensitive subjects did not react to this substance. Lanolin probably contains several sensitizers. Sensitization to lanolin in guinea pigs seems previously to have failed. In this study, a methanol extract of a lanolin preparation containing large amounts of sterols sensitized guinea pigs.